Over the twenty-year period from 1977 to 1997 the number of crimes recorded by the UK Home Office increased by over 75%, with crimes against property accounting for 91% of all recorded crimes in 1997. Personal property is therefore increasingly at risk of being stolen or interfered with at any time, particularly when left unattended or out of view. Effective methods of monitoring the security of personal property are therefore greatly in need.
An example of such a personal property alarm system is disclosed in GB-A-2228814 (Magrill). The Magrill system comprises a radio transmitter unit, retained by the property owner, and a radio receiver unit, fixed to the property to be secured. When in use, the transmitter unit emits a constant signal that is received by the receiver unit. Both the transmitter and receiver units incorporate motion detectors. This system is designed to alert the property owner, by way of a loud audible tone emitted from the receiver unit, if their property is moved and the signal received by the receiver from the transmitter falls below a predefined strength or is absent. While the strength of the signal received by the receiver remains constant no alarm sounds. If the transmitter unit remains stationary for a certain period of time, the transmitted signal is curtailed to conserve power. If the transmitter unit is subsequently moved, its motion detector re-activates the transmitter and signal transmission is resumed. While the receiver unit remains stationary no alarm sounds, even if the transmitter unit moves away from the property. If the receiver unit is subsequently moved, its motion detector re-activates the receiver and it awaits a signal from the transmitter. If no signal is received or its strength diminishes, the alarm sounds. The Magrill system allows manual adjustment of the motion detector sensitivity, duration and/or loudness of the alarm and operational range.
A further example of a personal property alarm system is disclosed in FR-2608868 (Estienne). The Estienne system comprises a transmitter unit in radio communication with a receiver unit. The transmitter emits a constant series of pulses to be received by the receiver. While the frequency of pulses remains at a predefined value, no alarm sounds. If the frequency of pulses received by the receiver changes, the alarm is triggered. The frequency rate expected by the receiver, and transmitted by the transmitter, can be manually adjusted. The operational range of the system is also adjustable.
Both the Magrill and Estienne systems both use a simple one-way transmitter/receiver radio communication monitoring system. The use of such a simple system restricts its operational flexibility and allows the security of only one item of personal property to be monitored per transmitter unit.